1. Field of the Invention
The present invention is directed to a circuit for switching a video signal to a reference level for controlling the brightness of the picture screen of a video output tube. Such circuits are known as clamp circuits.
2. Description of the Prior Art
The picture of a video output tube is constructed of individual lines, which are in turn composed of successive picture elements. These lines are scanned by an electron beam by applying appropriate control signals to the electron gun of a video output tube. The brightness of the picture elements of each line is controllable by applying a control voltage to a control grid of the video output tube. This control voltage, which is the video signal, contains the information for a line which is to be pictorially displayed. The image at the picture screen of the video output tube is derived from the total of all line signals. These video signals are broadband signals consisting of pulses separated by pulse pauses, with the maximum black signal, the "black value signal," being set at the picture screen of the video output tube given a small amplitude of the video signal, i.e., when the signal corresponds to a reference level. A bright (white) signal is set at the picture screen of the video output tube when the amplitude reaches a maximum value. Signal amplitudes in between these values are portrayed as "gray signals."
A conventional video signal is shown in FIG. 1 on the basis of which the image at the picture screen of the video output tube appears white in the upper image half and appears black in the lower image half. The period of the signal is referenced T.sub.s, with the signal consisting of individual pulses having respective chronological durations T.sub.w and T.sub.s. During the time T.sub.w, a white picture signal is generated, whereas during the time T.sub.s a black picture signal is generated. During a time T.sub.v, the electron beam is switched to "dark," i.e. to a reference level which corresponds to the "black value signal" and is returned to the start of the first picture line.
The same signal is shown with an expanded time frame in FIG. 2, wherein it can be seen that the signal in each duration T.sub.w and T.sub.s consists of a series of pulses separated by pulse pauses. Each pulse has a chronological duration T.sub.z, during which the electron beam is deflected along a line of the picture screen, and the electron beam is switched to "dark" during each pulse pause, i.e., during each chronological duration T.sub.H, and is driven to the start of the next line (retrace interval). During this time, the video signal is applied to the reference level which corresponds to the black value signal for "switching" the electron beam to "dark."
Due to their lower bandwidth limitation, video amplifiers which are usually used for this purpose are not capable of properly transmitting DC voltage components as well as voltages having low frequency. These voltages must then be reconstructed with appropriate circuits, known as clamp circuits. Without this correction, low frequency voltages are superimposed on the video signal which cause the reference level to vary dependent on the previous signal values upon each switch to "dark." This causes the basic brightness of each line to be slightly different, and line noise thus arises, which is undesirable. A basic clamp circuit is described, for example, in the book "Fernsehtechnik ohne Ballast," Limann, 12th Edition, page 257, FIG. 17.43.
FIG. 3 shows a schematic illustration of a circuit operating according to known principles for cancelling this undesired effect. Given optimum dimensioning of this circuit, the generator resistance of the first amplifier V.sub.1 should be optimally low, the resistor R in combination with the coupling capacitance C.sub.k should have a short time constant, and the internal resistance of the followers amplifier V.sub.2 should be optimally high. Undesirable low frequency signal components which are produced by the image content, and superimpositions of noise frequencies such as microphonics or radiated interference should be suppressed.
As can be seen in FIG. 4, noise influences due, for example, to the line deflection, occur particularly at the start of the pulse pause having the chronological duration T.sub.H in the video nal, so that the switch S shown in FIG. 3 can be driven only during a short time span at the end of this pulse pause. Drive of the switch S during the entire pulse pause can result in the capacitor C.sub.k not being completely discharged to the reference potential U. The reference levels at the start of the lines are thus different, thus causing line noise.
The drive pulse I.sub.A for the switch S is shown in FIG. 5. The chronological duration of this pulse must be such that the capacitor C.sub.k can accept the reference potential U, and thus can assume the desired initial value for the following line. To this end, the time constant of the circuit shown in FIG. 3 should be maintained optimally short, so that a full charge reversal of the capacitor C.sub.k can occur given high noise frequency amplitudes and given a single drive pulse I.sub.A.
Noise signals deriving from the video pre-amplifier V.sub.1, however, are superimposed on the video signal. These noise signals result, given extremely short drive pulses I.sub.A and given the low time constant of the circuit, in the capacitor C.sub.k not assuming the reference level, but instead assuming a voltage level which is modified by the momentary noise level. A reference potential falsified by the noise level is thus present for the next following line. This reference potential changes for each line due to the statistical change in the noise level, so that line noise again arises. For this reason, the time constant of the circuit of FIG. 3 is usually increased in comparison to the aforementioned, ideal case, so that the noise level can be averaged out during the pulse pause. A compromise must therefore be sought in the selection of the time constants of the circuit, so that the noise frequencies are still adequately suppressed and the line noise does not become visible.